My 1991 4Runner is slipping out of overdrive.......it slips out, and then back in overdrive when it is warmed up. The transmission shifts VERY smoothly, it just has this overdrive issue! When the 4Runner is cold, it will not shift into overdrive at all. Any suggestions? HELP!!

Check your vacuum hoses for cracks, poor connections, no connection. Replace any that look questionable.

Check your intake manifold vacuum. Poor vacuum (high pressure) may indicate a vacuum leak (bad gasket, bad hose).

There is another post (recent) which is exactly like yours. I have also experienced this problem. In the the other post and mine as well the problem turned out to be a bad thermostat. Simple problem but amzingly the transmission shift points are very tied into the temp of the engine. Also, replace the thermostat with a genuine Toyota one. I kept having problems, (went through 2 aftermarket thermostats in 3 months) and then then finally replaced it with a Toyota one (just a few bucks more) and have not had a problem with it jumping out of overdrive ever again. For me this used to happen mostly on the highway at speeds above 60mph.

Was there a noticeable problem with the thermostat (besides the overdrive problem) such as overheating or the temp going very hot to cold, etc? My temp seems to be fine. Thanks!

I too had a problem with my overdrive. It was the thermostat. I would replace that, since it's cheap to replace. I didn't have any issues with overheating or anything either

No, my temperature guage read in the normal range and there were no swings from hot to cold or anything else. Again though, please use the thermostat from your Toyota dealer as I would have been way ahead if I had followed this advice. Even though the garage that replaced the aftermarket thermostat did it for nothing after it went bad, the 3rd time I hand carried the geunuine Toyota thermostat that I had purchased to the independent garage who then installed it. Haven't had a problem since!!

No, my temperature guage read in the normal range and there were no swings from hot to cold or anything else. Again though, please use the thermostat from your Toyota dealer as I would have been way ahead if I had followed this advice. Even though the garage that replaced the aftermarket thermostat did it for nothing after it went bad, the 3rd time I hand carried the geunuine Toyota thermostat that I had purchased to the independent garage who then installed it. Haven't had a problem since!!


Thanks for the information.......I changed the thermostat and the difference is night and day!! My 4Runner has NOT shifted out of overdrive since replacing it....I have also noticed a jump in my miles per gallon as ALL the shift points have changed for the better! The thermostat is the last thing I would have thought of!! Many thanks!